Many ceramic (or ceramic composite) materials are used in the production of industrial cutting tools and components. Powders of these materials are typically pressed into shaped preforms which are then sintered at high temperatures (1000.degree. to 2000.degree. C. depending on the material) to densify and strengthen the tool or wear surface. Silicon nitride ceramics are particularly preferred for industrial cutting tools because of their high strength, fracture toughness, wear resistance and high temperature properties.
Ceramic materials are quite difficult to sinter to nearly full density. Hence a common manufacturing process includes "hot pressing", wherein a disc of the ceramic powder material of interest is pressed in a high temperature furnace using a mechanical press. The hot pressed disc is then sliced, diced or core drilled to obtain small ceramic work pieces of the desired shape and size. These are expensive processes.
In the conventional sintering processes, the preform of the ceramic powder is brought up to its sintering temperature in a radiant heat oven. In order to produce crack-free products, the sintering process is conducted with a slow heating rate. Furnace cycle times are in the order of many hours. The high temperatures and long heating times can lead to undesired decomposition in the ceramic materials being sintered.
Many ceramic materials are not capable of being sintered to the desired densities (typically greater than 98% of theoretical density). Expensive post sintering processes such as hot isostatic pressing are needed.
Most ceramic materials are "transparent" to microwave energy, that is microwaves can pass through them. As microwaves pass through the ceramic, some energy is absorbed by the ceramic body. This energy is converted to heat and is capable of heating the ceramic body volumetrically (uniform heating through the volume). Microwave heating of ceramics has many advantages which derive from a much more rapid heating rate. Higher heating rates can result in better densification. Rapid microwave heating can also reduce the ultimate temperature necessary to achieve densification. Improved rapid heating to lower ultimate temperatures can lead to the production of denser ceramic materials with finer grain size. These are important features in producing high strength, wear resistant ceramics.
In spite of the advantages to be gained by microwave sintering, there are several problems which have hindered its application with ceramic powders. Many ceramic materials do not couple well with microwave radiation at low temperatures, that is they are poor microwave susceptors below about 500.degree. C. Thus, to apply microwave energy for sintering, many ceramics need to be preheated by conduction, convection or radiation from another source such as a flame or a heating element, or a microwave susceptor material which couples with the microwave radiation, at least until a high enough temperature is reached, after which the ceramic couples with the microwave radiation. When microwave susceptors are used as a packed bed around the ceramic or metal materials to be sintered, uneven heating is often experienced. Some microwave susceptors, such as carbon, become conductors at higher temperatures, which can lead to uneven heating or arcing. Also, as the ceramic is sintered, it shrinks due to densification, and can lose contact with the susceptor bed. Volume shrinkage during sintering is usually about 50 percent. Many microwave susceptors may themselves sinter or fuse together in the susceptor bed, leading to uneven or inefficient sintering of the product. Still other microwave susceptor materials may decompose, contaminate or react with the material to be sintered.
Canadian Patent Application 2,000,109 of Apte et al., laid open on Apr. 3, 1991, describes a microwave sintering process for certain non-susceptor materials such as alpha alumina in a powder bed of susceptor materials such as sub-alpha alumina. Canadian Patent Application 2,001,062 of Apte et al., laid open on Apr. 19, 1991, discloses a microwave sintering process for sintering certain ceramics including silicon carbide, silicon nitride and aluminum nitride. A packed powder bed consisting of a microwave susceptor (ex. metal carbides, carbon, porcelain, soda-lime glass and barium titanate), an oxygen getter (ex. metal carbides, carbon and oxidization metals), a thermal conductor (ex. boron nitride, aluminum nitride and metals), and a protective material to generate a localized protective atmosphere (ex. metal carbides, carbon, MoS.sub.2, lead based ceramics). These and other prior art approaches to microwave sintering of ceramics in powder beds still present problems:
1. Many of the prior art processes utilize complex microwave susceptor beds wherein the materials are chosen to, in situ, form and maintain a controlled, protective atmosphere during sintering. Thus, for sintering of silicon nitride, the packed bed might contain silicon nitride. However, using a solid nitride to provide a protective nitrogen atmosphere is problematic since the silicon nitride powder in the bed decomposes to release nitrogen at the same temperature as the silicon nitride ceramic piece also starts to decompose. The oxygen available at lower temperatures will thus oxidize the ceramic pieces. PA0 2. Many of the powder susceptor beds themselves sinter during the sintering process, creating large gaps in the bed, and uneven or inefficient heating. PA0 3. The use of a packed powder bed to prevent oxygen entering the bed during sintering necessitates a careful, time consuming packing step. Oxygen trapped in the bed is available for oxidizing the work pieces. PA0 4. The use of other materials such as silicon carbide or carbon, as the main ingredients of a microwave susceptor bed is problematic. These materials become good electrical conductors, and thus poor microwave susceptors, as the temperature increases during sintering. They can also shield the ceramic pieces from the microwave field, i.e. prevent microwave energy from reaching the ceramic pieces. PA0 5. Many of the materials suggested for use as microwave susceptor bed ingredients are expensive ceramics (ex. silicon nitride and boron nitride.).
One prior art approach to microwave sintering of ceramics is to use higher frequency microwaves (see for example U.S. Pat. No. 4,963,709, issued Oct. 16, 1990, to Kimrey et al.). At these higher frequencies (ex. 14, 28 and 60 GHz), the ceramic material couples with microwaves, for direct sintering. However, the cost of high frequency, specialized microwave equipment is prohibitive for most ceramic sintering applications. At the commonly used frequencies (915 MHz and 2.45 GHz) equipment is relatively inexpensive and readily available.
There remains a need for an effective microwave sintering process to sinter ceramic and ceramic composite materials.